In air bag systems there is typically a cushion or bag which, upon a collision, is inflated to provide protective cushioning to the passenger. To inflate the cushion such systems employ an inflator to produce a quantity of inflation gas. Various inflators operating upon different principles are known. For each operating principle, there is a slight difference in performance such as the rise and fall of pressure in the cushion over time. As such, for different applications, inflators operating on different principles are preferred.
For air bag systems to be effective they must be activated immediately at the moment of collision and a proper inflating time is an absolute necessity. On the other hand, if the air bag inflates too rapidly, the passenger may be injured by the inflating air bag. For example, assume that a child is standing on the passenger seat in front of the instrument panel. If the air bag is activated, the sudden and rapid inflation of the bag could throw the child off the seat and may cause injury to the child. In order to prevent such injury by the inflating air bag, it is necessary that the inflating speed of the bag be controlled. Therefore, a multi-stage air bag device has been proposed in order to satisfy these contradictory requirements as described above.
The multi-stage air bag device works such that the air bag is inflated to a predetermined level during the initial stage of the collision and after a brief interval it is inflated completely. In prior air bag devices, a predetermined amount of gas is generated in the initial stages of collision so as to inflate the air bag to some degree. A great quantity of gas is then generated after an interval established somewhere between several milliseconds to several tens of milliseconds. With such a multi-stage air bag device, the chances of injury to an out-of-position passenger can be reduced and a wider range of crash conditions can be accommodated.
The time intervals between the initial stage deployment and the subsequent stage deployments are set in relation to the quantity of gas used in the first stage and other factors. However, an improper interval adversely affects the air bag as a safety device. For example, if the interval is too short, it may cause injury, but if the interval is too long, the shock absorption action of the air bag will be weakened and may result in injuries to the vehicle occupant. For this reason, a high degree of precision is required for the delayed ignition signal generator whose function it is to activate the first gas generant and thereafter the second and third gas generant.
A typical delayed ignition signal generator is made up of complex electronic circuitry. Because of the complexity of the system and the attended high cost, its applicability to such infrequently used devices such as air bags, is limited. While such adaptive systems are desirable, they typically require additional components on the inflator. Further, it is even more difficult to provide an adaptive pyrotechnic inflator which will meet the size requirements for vehicles, especially for driver side and side impact applications.